This invention relates to an antenna that is capable of communicating with both a satellite system and a terrestrial system simultaneously. For example, the antenna may be conveniently used to receive signals broadcast by a direct broadcast satellite radio system or other high altitude broadcast system, in which radio or other signals signals are broadcast directly from one or more satellites to mobile vehicles on or near the ground and are also received by terrestrial repeaters, and then rebroadcast terrestrially to the mobile vehicles on or near the ground.
Satellite-based direct broadcast systems are currently used to broadcast TV and radio signals to fixed ground stations which typically use a dish-shaped antenna to receive the signals. These systems have become very popular and soon this direct broadcast satellite technology is moving into the vehicular field. Vehicles pose a number of interesting challenges for this technology. First, in the case of terrestrial vehicles which can move on or near the surface of the earth, their movement means that the satellite signal will be occasionally blocked due to natural and man-made obstructions near which the vehicles travel. Since the satellite signals can be blocked by obstructions such as buildings and mountains, it has been proposed to transmit a second signal terrestrially which is locally provided by a repeater located to receive the satellite or high altitude broadcast signals without interference. See FIG. 1. The direct broadcast satellite signals will arrive at the vehicle 1 with circular polarization from a location possibly high above the horizon due to the altitude of satellite 2. In contrast, the repeated signals will arrive with vertical polarization from a repeater location 3 frequently near the horizon. Services which will be using such technology include possibly XM Radio and Sirius Radio. The entire frequency range allocated for XM Radio is 2.3325 to 2.345 GHz, and the entire frequency range allocated for Sirius Radio is 2.320 to 2.3325 GHz. This includes the satellite signal as well as the terrestrial signals from the repeaters. The total bandwidth required is much less than the bandwidth of the antenna disclosed herein.
Using conventional antenna technology, the antennas on a vehicle 1 to receive such signals would tend to be (i) numerous, (ii) unsightly and/or non-aerodynamic, (iii) possibly expensive, and (iv) would be difficult to point properly.
Similarly, as demand for existing wireless services grows and other new services continue to emerge, there will be an increasing need for still more antennas on vehicles. Existing antenna technology usually involves monopole or whip antennas that protrude from the surface of the vehicle. These antennas are typically narrow band, so to address a wide variety of communication systems, it is necessary to have numerous antennas positioned at various locations around the vehicle or to complicate the antenna design by making them multiband antennas. Furthermore, as data rates continue to increase, especially with 3G, Bluetooth, direct satellite radio broadcast, wireless Internet, and other such services, the need for antenna diversity will increase. This means that, if conventional antenna technology is followed, each individual vehicle would require multiple antennas each operating in different frequency bands, and/or with different polarizations and sensitive at different elevations relative to the horizon. Since vehicle design often dictated by styling, the presence of numerous protruding antennas will not be easily tolerated.
With the increasing number of wireless data access systems that will be incorporated into future vehicles, the number of antennas is also apt to increase. Many of these new data access systems will involve communication with a terrestrial network and also with a satellite or other high altitude transmitter. One such system is the previously mentioned direct broadcast satellite radio which will soon be operational. Transmitting systems aboard satellites typically broadcast in circular polarization so that the receiving mobile vehicle can be in any orientation with respect to the satellite, without the need to orient the vehicle""s antenna. However, terrestrial broadcast systems typically use linear polarization for multi-path reasons, with vertical polarization being preferred for moving receiving stations for reasons well known in the art. Hence there is a need for antennas which can receive both circular polarization from the sky as well as vertical linear polarization near the horizon. These antennas exist, with the most common example being the helix antenna. One disadvantage of the helix antenna is that it protrudes one-quarter to one-half wavelength from the surface of the vehicle. Since current direct broadcast radio systems operate at 2.34 GHz, this results in an antenna that is several centimeters tall. The presence of an unsightly vertical antenna and/or a plurality of antennas, is often unacceptable from a vehicle styling point of view. Additionally, such antennas increase the aerodynamic drag of the automobile which is undesirable for energy-conservation reasons.
As a consequence, there is a need for an antenna that can perform as well as the vertical helix antenna, but has a low profile so that it can easily be adapted to conform to the roof over the passenger compartment of a vehicle, for example. The antenna should preferably be simple to manufacture using common materials. The antenna should be capable of receiving signals having circular polarization from orbiting satellites as well as signals having vertical linear polarization from terrestrial stations or repeaters.
In the design of antennas for low-angle radiation, one must consider each section of the radiating aperture and how it contributes to the overall radiation pattern. If one restricts the antenna design to one having a low-profile (for example, an antenna having a thickness much less than a quarter wavelength), there are only a few fundamental elements available. The most common low-profile antenna is the patch antenna, which is shown in FIG. 2. The patch antenna consists of a metal shape 10 supported above a ground plane 12 and fed by a coaxial probe or other feed structure 14. While the patch is a common low-profile antenna element, it is a poor choice for receiving (or transmitting) radiation at low angles. The reason for this is that the two edges 10-1, 10-2 of the patch 10 both radiate and the interference between the two determines the overall radiation pattern of the antenna. In the direction normal to the ground plane 12, the interference is constructive and the patch 10 provides significant gain in that direction. However, in a direction toward the horizon (e.g. in a direction parallel to the ground plane 12), the interference is destructive, and the patch produces very little radiation in that direction. One way to avoid this problem is to bring the two edges 10-1, 10-2 of the patch closer together. However the effective overall length must remain one-half wavelength, so this requires that the patch be loaded with a high dielectric material. Furthermore due to the difficulties of achieving very high dielectric materials, there is a limit to how small a patch can be. Moreover, as the patch size is reduced, its bandwidth is also reduced.
A unique feature of the preferred embodiments of antenna disclosed herein is that it can receive both circularly polarized signals from a satellite in the sky as well as vertical linearly polarized signals from a terrestrial repeater. For the purpose of this specification and the claims herein, the term xe2x80x9csatellitexe2x80x9d is defined to mean an object which is in orbit about a second object or which is at a sufficiently high altitude above the second object to be considered to be at least airborne and xe2x80x9cterrestrialxe2x80x9d or xe2x80x9cearthxe2x80x9d is defined to mean on or near the surface of the second object.
An advantage of the present invention is it can achieve these properties with a form factor that is much thinner than one-quarter wavelength in height, and only slightly larger than one-half wavelength square in area. Indeed, the height of the antenna is preferably under 5% of a wavelength.
Since the antenna form factor is very important to vehicle designers, the small package permitted by this antenna is preferable to other competing designs which typically involve protruding antenna elements that are one-quarter wavelength in height or taller. For upcoming direct broadcast satellite radio systems, this translates into an antenna height of several millimeters (mm) for the antenna disclosed herein compared to several centimeters for competing designs.
The most significant antenna problem for a direct broadcast satellite signal receiving system as shown by FIG. 1 is communicating with a terrestrial network, because this involves receiving radiation from low angles, across the metal roof of a vehicle, in addition to receiving signals directly from satellites. Typically this requires that the antenna have significant height, or that it be elevated above the ground plane. The present antenna achieves this unique form factor by utilizing a slot antenna which has a good fundamental geometry for receiving at low angles. This is because a single slot antenna has only one radiating aperture, which is the thinnest possible aperture for a given wavelength. Furthermore, a slot antenna generates the greatest currents in the surrounding ground plane which are responsible for radiation to low angles.
The preferred embodiments of the present antenna involve a crossed pair of slots which are slightly detuned from one another in order to generate circular polarization for satellite reception. Thus, this antenna achieves good performance for both satellite reception and terrestrial reception, in a very thin design.
The present invention also provides a unique feed geometry, which allows the antenna to be fed at only one location, and represents a significant improvement over existing designs. Optionally it includes a radome structure, and the capability for active electronics such as amplifiers to be included in the antenna package.
The antenna described below achieves these features and other in a volume that is only a few millimeters tall. While the specific embodiment of this antenna discussed below is specifically designed for a direct broadcast satellite radio system, it can also be applied to other systems involving communication with both a satellite and a terrestrial network.
In one aspect, the present invention provides a crossed slot antenna having a resonance frequency, the antenna comprising an electrically conductive structure defining a cavity therein; first and second slots formed in the electrically conductive structure, the slots having different lengths such that one slot has a resonance frequency above the center frequency of the antenna and such that the second slot has a resonance frequency below the center frequency of the antenna; and a common feed point which is arranged to couple the radio frequency signal from the slots to said common feed point.
In another aspect, the present invention provides a method of fabricating a crossed slot antenna comprising the steps of: (a) forming a cavity using a printed circuit board plated with metal on opposed surfaces; (b) etching two slots in the plated metal, the slots having slightly different lengths and intersecting each other at a 90 degree angle; and (c) forming a metal plated via in said printed circuit board, said metal plated via defining a common feed point for the slots.
In still another aspect, the present invention provides a method of fabricating a crossed slot antenna comprising: (a) forming a cavity structure having conductive material on opposed surfaces thereof; and (b) etching two slots in the conductive material, the slots having slightly different lengths and intersecting each other at approximately a 90 degree angle.
In another aspect, the present invention provides a crossed slot antenna comprising: (a) a cavity structure having conductive material on or forming opposed surfaces thereof; and (b) two slots in the conductive material, the slots having slightly different lengths and intersecting each other at or close to a 90 degree angle.
The present invention, in yet another aspect, provides a slot antenna having: (a) a cavity structure having conductive material on or forming opposed surfaces thereof; (b) at least one slot in the conductive material on a first surface of the cavity structure; and (c) a feed point for the slot, the feed point being disposed in and penetrating the cavity structure, the feed point being coupled to the first surface at a point thereon which is spaced from the slot.
In still yet another aspect, the present invention provides an antenna unit for mounting on a vehicle, the antenna unit comprising: (a) a support surface and a mounting device for mounting the antenna unit on the vehicle; (b) an antenna adapted for receiving circularly polarized radio frequency signals in at least directions oblique to the support surface; and (c) a protective cover for the antenna.
The present invention, in yet another aspect, provides a method of receiving circularly polarized radio frequency signals comprising the steps of: (a) providing a slot antenna having two slots which cross each other in a surface of a cavity structure; (d) varying the lengths of the slots so that the slots have different individual resonance frequencies; and (c) providing an antenna feed point on the surface which is spaced from both of the slots.
In a different aspect, the present invention also provides a method of designing a crossed slot antenna capable of receiving both circularly polarized radio frequency signals and linearly polarized radio frequency signals, the crossed slot antenna having a pair of crossed slots formed in a surface of a cavity structure. The method comprises the steps of:
(a) calculating an effective dielectric constant in the slots of the crossed slot antenna that is the average of dielectric constant of the cavity and that of any radome or other environment located above the slots;
(b) calculating an effective index of refraction n, where n={square root over (∈average)} and where ∈average=the dielectric constant calculated in step (a);
(c) determining an initially calculated average length of the slots of xcex/2n where xcex=the wavelength of a desired resonance frequency of the crossed slot antenna;
(d) calculating an inherent bandwidth of crossed slot antenna based on the formula 6xcfx80V/xcex3 where V=the volume of the cavity structure;
(e) determining an initially calculated length of each slot by adding, for one slot, and subtracting, for the other slot, a distance equal to one-half of the inherent bandwidth, expressed as a percentage, of the antenna;
(f) adjusting the initially calculated length of each slot by experiment.